1. Field of the Invention
The present invention relates to the field of manufacture of golf club shafts, particularly of aluminum or aluminum alloys which must have a minimum defined stiffness and which benefit from weight reduction. As referred to herein, the term "defined stiffness" for golf shafts refers to a measured vertical deflection of the tip end of a shaft from which a weight is suspended when the butt or handle end of the shaft is clamped to horizontally support the shaft in cantilever fashion. The industry defined S, R, L, X stiffiess scale for golf shafts is well known. Defined stiffnesses of other sports articles are also known or are easily determinable.
2. Prior Art
Tubular sporting articles such as baseball bats and golf club shafts made of metal materials such as aluminum alloys which have a maximum modulus of elasticity of about 10.4 are well known. Throughout this disclosure, elastic (Young's) modulii expressed for example by the number 11 will be understood by persons skilled in the art to mean 11.times.10.sup.6 psi.
As defined herein, the term metal matrix composite (MMC) refers to a metal or metal alloy having an undissolved portion of non-metal reinforcing fibers, platelets or particles uniformly dispersed therein. MMCs comprising alloys of metals such as aluminum reinforced with non-metal fibers or particles such as ceramic particles are known and, although their use has been broadly suggested for golf shafts, the usual reinforced aluminum alloys typically have elastic modulii significantly in excess of about 13 and may be formulated to have elastic modulii as high as 20 or 30 or even above pending upon the end use of the products for which they are intended. These MMC modulii are considered excessive and thus inherently unsuitable for golf shafts.
The elastic modulus of MMCs increases as the volume percent of reinforcing fibers such as carbon, silicon carbide or boron fibers or platelets or particles of ceramic, e.g., aluminum oxide, silicon carbide, etc. in the product increase from about 15% to 40% by volume. These MMC materials are of approximately the same density as or slightly higher density than non-reinforced alloys but are considerably stiffer, e.g., from 30 to 50 percent stiffer, than the same un-reinforced aluminum alloy. On the other hand, the tensile yield strength of aluminum alloy MMCs increases relatively insignificantly (less than 10%) over that of un-reinforced aluminum alloys despite the added non-metal reinforcement. Unlike alloying elements that dissolve in molten aluminum, the added reinforcing platelets, fibers or ceramic particles in MMCs remain in platelet, fiber or powder form with no significant chemical reaction. MMCs may therefore be generally categorized as continuous reinforced alloys or as discontinuous reinforced alloys. Continuous reinforced alloys employ strands or fibers for the reinforcement whereas discontinuously reinforced alloys use reinforcement in particulate or platelet form.
Continuously reinforced alloys or MMCs employing silicon carbide fibers have been suggested for use in tubular sports articles such as bicycle frame parts which require light weight and substantial stiffness. Continuously reinforced MMCs have not heretofore been found acceptable for commercial use in shaped articles such as golf club shafts further because of relatively poor workability characteristics of continuously reinforced MMCs. Mechanical workability is essential to obtain the desired shaft shapes without sacrifice of acceptable strength, flexibility, light weight and good fatigue resistance. MMC technology has generally emphasized the addition of substantial proportions of reinforcing fibers or powders to the matrix alloy to obtain substantially greater stiffness. This has resulted in MMCs which are inadequately drawable and thus unsuitable for formation of tubular shapes such as golf shafts which not only must have a tapered configuration with thin walls for light weight but must be reformed from the original tubular shape to form an enlarged cylindrical butt or handle end and a re-shaped short cylindrical tip end. Additional variations in the shaft wall thickness to create a kick point of maximum shaft flexibility at a desired position or to form the more recently introduced "bubble shaft" configurations having an enlarged section proximate the lower portion of the butt end of the shaft require additional steps in the forming process. Also, MMCs work harden relatively quickly which makes tapering of articles such as golf club shafts very difficult.
Various MMCs have been extensively studied but golf shafts manufactured therefrom for test purposes have previously proven unsuitable. U.S. Pat. No. 4,702,770 issued Oct. 27, 1987 to Pyzic, et al. is one example representative of boron carbide aluminum composite technology.
Accordingly, improved tubular shaped metal sporting articles having a defined stiffness and reduced weight due to a reduction in wall thickness, and with adequate strength despite this reduction in wall thickness are always desired. Shaped metal sporting articles such as golf shafts with high strength-to-weight ratios without sacrificing flexibility, torsional resistance or fatigue resistance and possessing workability properties required for economy of manufacture and ease of golf club assembly and repair are particularly desirable.